1. Field of the Invention
This invention relates to the art of recording and more particularly, to thermosensitive recording material which is capable of recording an image when it is heated and the once recorded image can be retained as it is over a long period of time even if brought into contact with water, oils, plasticizers or organic solvents.
2. Description of the Prior Art
Typical thermosensitive recording papers are made by applying, onto a porous support such as a paper sheet, a paint of a leuco dye, an acidic material and a binder therefor and drying the paint to form a thermosensitive layer on the support. The thermosensitive paint tends to penetrate into the porous sheet, so that the resulting thermosensitive coloring layer becomes irregular with regard to the thickness. When such a thermosensitive recording sheet is subjected to thermosensitive recording when it is heated such as, for example, letters or a bar cord, an illegible pattern results. Especially, when a bar cord is recorded, the respective lines become discontinuous, showing the tendency that the bar cord is erroneously read. To avoid this, it is the usual practice to apply the paint in large amounts and, after drying, to calender the resulting coloring layer so as to increase the smoothness of the layer. However, these procedures lead to a lowering of the quality of the thermosensitive recording material. In particular, the calendering may result in an undesirable color development of the thermosensitive coloring layer, thus lowering the brightness of the layer.
It is a still further object of the present invention to provide a thermosensitive recording material having a thermosensitive coloring layer in which a high quality coating is uniformly effected.
It is a still further object of the present invention to provide a thermosensitive recording material effecting good adherence between the thermosensitive coloring layer and an under coating layer comprising a polyolefin resin layer and a support sheet.
Although the known thermosensitive recording sheet has a barrier layer of a water-soluble polymer material on the side opposite to the coloring layer in order to prevent the disappearance of a recorded image, the barrier layer cannot be resistant to water when immersed in water over a long period of time and thus the thermosensitive layer may be peeled off. This is true even if the water-soluble polymer material is improved in water resistance by application of a hardening agent thereto.
As is known in the art, one of the main applications of these thermosensitive recording materials is a thermosensitive recording adhesive label. The adhesive label is used for attachment to commercial articles in order to indicate prices, article and shop names, and the like. In these labels, the above-described drawbacks are also involved.